Conventionally, a core drill is widely used for drilling a hole in concrete and the like. This core drill is generally of the construction in which a drive mechanism for generating a rotary power is coupled via a shank to a core body having cutting blades such that, when driven by the drive mechanism, the shank and the core body are rotated integrally. Such a construction is disclosed, for example, in Japanese Patent Application Unexamined Publication No. 2004-34210 (Patent Document 1).
Furthermore, core drills can be classified into wet type and dry type, and in the case of a core drill of a wet type, water is fed during drilling a hole by a water feed apparatus coupled to the core drill so as to cool the cutting blades provided on the core body. Specifically, a transverse hole is formed in a shank coupled to the core body to pass through the shank in a direction perpendicular to its longitudinal direction (radially). The shank is further formed with a vertical hole that extends lengthwise and through the rotational center axis of the shank, which vertical hole is connected at one end thereof to the above transverse hole and opens at the other end thereof at the end portion of the shank on the core body side. Thus, if cooling water is supplied from the water feed apparatus externally mounted on the core drill, the cooling water passes through the above transverse hole and vertical hole of the shank to be sent to and cool the cutting blades on the core body.
As an example of a water feed apparatus, there is one that includes a relatively large-sized stationary water tank and pressure pump, with the water tank being coupled to the core drill by means of a hose. In the case of this water feed apparatus, compressed air can be introduced into the water tank by the pressure pump, and cooling water is fed with a pressure to the core drill by expansion of the compressed air.
As another example of a water feed apparatus, there is one of a natural fall water feed system that includes a water-pouring cup with a water outlet located at a lower portion thereof. In the case of this water feed apparatus, the cooling water stored in the water-pouring cup is sent out through the water outlet under the action of gravity, and is fed through the transverse and vertical holes formed in the shank of the core drill to the core body.    Patent document 1: Japanese Patent Application Unexamined Publication No. 2004-34210.